OLED (organic light emitting diode) devices are commonly known, so a detailed explanation is not necessary here. Suffice it to say that an OLED comprises a layer of a special type of polymer or small molecules, arranged between a cathode layer and an anode layer. When a voltage is applied between these cathode and anode layers, the intermediate OLED layer emits light (in contrast to an inorganic LED on the basis of PN junctions, which typically behaves as a point source).
FIG. 1A is a graph illustrating the current (vertical axis) versus voltage (horizontal axis) characteristic of an OLED having normal behavior. When the device is OFF, the voltage is zero and the current is zero. When the device is switched ON, the voltage rises and so does the current. The precise shape of the current/voltage curve may be device dependent, but in general the current is neglibly small in a first voltage range (in the example of FIG. 1A, the current remains below 0.1 μA for a voltage from zero up to about 2.5 V), and then the current quickly rises to reach a value of about 1 mA at about 4 V, when the device is considered to be ON. A device showing such normal behavior will in the context of the present invention be indicated as a “healthy” device, and it will be considered to be in a “healthy condition”.
A problem with OLEDs is that an OLED can be in a faulty condition; such device will in the context of the present invention be indicated as a “faulty” device. FIG. 1B is a graph similar to FIG. 1A, illustrating the current versus voltage characteristic of a faulty OLED. For voltages higher than the first voltage range, there is no visible difference, but for voltages in the first voltage range the current can become substantially higher, for example a few to several tens of times, in extreme cases (as illustrated in FIG. 1B, curve 2) even in the order of 100×-1000×: depending on the driving history of the device, in particular in the range from 0 to 2.5 V, an OLED device may switch from a healthy state (curve 1) to a faulty state (curve 2). In the following, the current for a faulty device will be indicated as “faulty current” while the current for a healthy device will be indicated as “healthy current”.
It is noted that, at least in principle, any OLED can make a transition from a healthy state to a faulty state. The difference between faulty current and healthy current level may differ between different OLEDs. In the example of FIG. 1B, the faulty current is lower for voltages just above the first voltage range as compared to voltages in the first voltage range, but this does not necessarily apply to all OLEDs.
It is further noted that, in practice, an OLED is either ON or OFF, and it will be in the transition from ON to OFF or back for only a very brief moment of time. Thus, at first sight it may seem that the problem is not severe, since in the ON state the current is the same for a faulty device. However, when an OLED is in its faulty state, its lifetime can be reduced substantially. This effect is believed to be caused by the fact that the current is not evenly distributed over the device's surface but is flowing only locally, leading to very high local current densities capable of locally destroying the device.
The present invention aims to increase the reliability and lifetime of OLEDs.
Whereas the above-described effect can be compared to some kind of short-circuiting in the device, one solution might be to remedy the short-circuit location after it has already occurred. However, this will most likely lead to a defective spot in the device (dark spot). In contrast, the present invention tries to prevent such short-circuiting from occurring, or at least reduce the chances on its occurrence.